Simultaneous production of higher chloromethanes

ABSTRACT

The higher chloromethanes, i.e., CH 2  Cl 2 , CHCl 3  and CCl 4 , are simultaneously produced by chlorinating methyl chloride with chlorine in a first reaction zone A, chlorinating at least one of the higher chloromethanes CH 2  Cl 2  and CHCl 3  with chlorine in a parallel second reaction zone B, combining the reaction products from said first and said second reaction zones A and B, separating higher chloromethanes from said combined reaction products, and recycling at least one of said separated higher chloromethanes CH 2  Cl 2  and CHCl 3  as chlorination feed to said second reaction zone B.

This application is a continuation of application Ser. No. 773,927, filed Sept. 9, 1985 which is a continuation of application Ser. No. 619,357 filed June 11, 1984 both now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the preparation of higher chloromethanes by chlorination of methyl chloride, and, more especially, to the simultaneous such preparation of select amounts of the three higher chloromethanes CH₂ Cl₂, CHCl₃ and CCl₄.

2. Description of the Prior Art

Conventional processes for the preparation of higher chloromethanes by chlorination of CH₃ Cl, as a practical matter, result only in the production of CH₂ Cl₂ and CHCl₃. The CCl₄ co-product constitutes less than 15% of the total production and is thus considered to be only a by-product of the reaction.

The production of CCl₄ is typically carried out by so-called "Tetra-per" (carbon tetrachloride/perchloroethylene) processes from chlorinated residues and/or saturated or olefinic hydrocarbons having two or three carbon atoms. However, such techniques lead to the generation of a not inconsiderable amount of heavy by-products. In addition, they consume large amounts of energy.

Because of these disadvantages of conventional processes, the preparation of CC1₄ from chloromethanes originating from the chlorination of CH₃ Cl would be of interest, especially since the most recent improvements in this process permit free radical chlorination in the liquid phase at a low temperature, using a chemical or photochemical initiator. Such processes are economical due to the high yields obtained and the simplification of the separation operations, the amount of troublesome by-products being small.

However, in the current state of the art, the simultaneous production of prescribed amounts, which may be close to one another, of the three higher chloromethanes does not proceed without giving rise to problems in the conventional apparatus which consists of a reactor for the chlorination of CH₃ Cl followed by downstream columns for purifying and separating the various products of reaction. Such a system lacks flexibility. In fact, in this type of unit, the principal constraint is the adjustment for the respective productions of CH₂ Cl₂ and CHCl₃ ; the amount of CCl₄ simultaneously produced is a function of the CH₂ Cl₂ -CHCl₃ proportion effectively obtained and the operating conditions. This production of CCl₄ can only be low with respect to that of the other two chloromethanes.

The production of prescribed amounts, which may be closed to one another, of the three higher chloromethanes in conventional apparatus comprising a single chlorination reactor must necessarily be balanced by the total amount of investment required for profitable operation. In fact, in this type of unit, the adjustment in the flexibility is obtained by limiting the degree of conversion of CH₃ Cl and, if necessary, by appropriate recycling of CH₂ Cl₂. This manifests itself in a significant over-dimensioning in the entirety of the apparatus concerned, and concomitant marked increase in operating costs. Moreover, the productivity of a given reactor, all things otherwise being equal, is sensitive to the composition of the higher chloromethanes produced. It is known that the rate of chlorination of a chlormethane substantially decreases as the number of chlorine atoms contained in the molecule increases: a reactor having a productivity of 100 for respective compositions of CH₂ Cl₂, CHCl₃ and CCl₄ of about 45/45/10% by weight would only have, all things otherwise again being equal, a productivity of about 65 for a distribution of 37.5/47.5/15 and of 30 for 20/45/35. "Productivity" is defined as the amount of chlorine reacted per unit volume of the reactor for a fixed level of unreacted chlorine at the outlet of the reactor.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision of improved chlorination apparatus/process, said apparatus comprising two reactors A and B in parallel, with reactor A operating in conventional manner for the chlorination of methyl chloride and reactor B, which is fed with chlorine and CH₂ Cl₂ and/or CHCl₃ produced in reactor A, serving to produce either the higher chloromethanes CHCl₃ and CCl₄ simultaneously, or CCl₄ by itself, and to ensure high flexibility respecting the overall apparatus.

Briefly, in the subject process adjustment in the production of CH₂ Cl₂, CHCl₃ and CCl₄ is effected by adapting, on the one hand, the distribution and amount of products formed in reactor A and, on the other hand, the production and the ratio of the amounts of CH₂ Cl₂ and CHCl₃ converted in reactor B.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE of drawing is a schematic/diagrammatic illustration of apparatus suitable for carrying out the process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

More particularly according to the present invention, with reference to the accompanying FIGURE of drawing, the reactor A is fed with Cl₂ and CH₃ Cl, respectively, via the lines 1 and 2. The reaction products are transferred via line 3 into a separation unit D: HCl/CH₃ Cl/higher chloromethanes, with HCl being removed via line 4 and CH₃ Cl and the higher chloromethanes being conveyed via line 5 into the separation unit E. Unreacted CH₃ Cl is removed via line 8 or recycled via line 6 to reactor A. In order to limit the temperature at the base of the tank of unit D, it is also possible to recycle more or less CH₃ Cl via line 7. The higher chloromethanes are conveyed via line 9 into the separation unit F for separating CH₂ Cl₂ from the other higher chloromethanes, it being possible for all or some of the CH₂ Cl₂ to be recovered at 12 or for all or some of it to be recycled via line 13 into reactor B. Finally, the remaining higher chloromethanes are conveyed via line 11 into the separation unit G for separating CHCl₃ from crude CCl₄, it being possible for all or some of the CHCl₃ to be recovered at 16 or for all or some of it to be recycled via line 17 into reactor B, the crude CC1₄ being recovered at 15. Provision may be made for recycling the higher chloromethanes into reactor B via lines 10 and 14. In this case, CCl₄, which does not interfere with the reaction products, is recycled into reactor B. This CCl₄ thus plays the role of diluent for the reactants and in this manner may decelerate the reaction; as a result, this recirculation should preferably be limited.

Before the higher chloromethanes are introduced into reactor B, it may be useful to first circulate them into a gas-scrubbing device C which enables the gaseous phase issuing from reactor B to be scrubbed.

The higher chloromethanes react, in reactor B, with Cl₂ introduced via line 18. The reaction products are combined via line 19 with those of reactor A in the simplest manner, in the separation unit D.

It is recommended, although this is not essential, that the chloromethanes being charged to reactor B contain as little CH₃ Cl as possible, such that this reactor at the same time maintains an acceptable selectivity and productivity for the given degrees of conversion of CH₂ Cl₂ and/or CHCl₃.

The adjustment in the amounts of CH₂ Cl₂ converted and CHCl₃ converted or produced is effected by adaptation of the composition of CH₂ Cl₂ and CHCl₃ in the reactor, as well as the rate at which the reactor is fed. The reaction parameters of each of the reactors, and of course the reactors themselves, are adapted such that the reactions take place in the liquid phase. The pressures in each reactor are in general close to one another and should be selected such that the separation of HCl and the chloromethanes is facilitated. Good results are obtained using temperatures in the reactors ranging from 50° to 120° C., preferably from 60° to 90° C., under pressures of 10 to 50 bar, preferably 15 to 25 bar. The ratio in reactor A between the number of molecules of Cl₂ and the number of molecules of CH₃ Cl advantageously ranges from 0.3:1 to 1.5:1, preferably from 0.5:1 and 1:1. The amount of Cl₂ introduced into reactor B is selected as a function of the amount of each of the higher chloromethanes introduced and of the higher chloromethane or chloromethanes desired to be ultimately obtained. The chlorination reactions in the reactors A and B are known per se, and are usually reactions of the free radical type, with initiation thereof being by means of a chemical or photochemical initiator.

The reaction products exit the reactors A and B in liquid or gaseous form. It is possible, although this is not essential, to scrub the gaseous phases with higher chloromethane in order to recover the trace amounts of chlorine which are contained therein and which constitute most of the unreacted chlorine. This scrubbing is of particular interest with respect to the products of reactor B. Such a scrubbing enables the excess chlorine to be recovered and as a result thereof significantly increases the productivity of the reactor.

The various reaction products of the reactors A and B are separated from each other in the downstream separation units, described above, under the typical distillation conditions appropriate for each of such products.

The process according to the invention has the advantage of concentrating the problems of limitation in productivity in a single auxiliary reactor B, which very substantially obviates same by reason of the fact that the reaction material in this reactor is formed from CH₂ Cl₂ and/or CHCl₃. Another advantage concerns the flexibility, which is particularly significant in the reaction unit and which preserves, in addition, all the economic advantages of the conventional process. In fact, for a given production of CCl₄, which may be up to 50% of the total production, the CH₂ Cl₂ -CHCl₃ distribution in the complement of the production may vary continuously over wide ranges, and more particularly, from an economic point of view, between 75/25 and 25/75% by weight.

In order to further illustrate the present invention and the advantages thereof, the following specific example are given, it being understood that same are intended only as illustrative and in nowise limitative.

EXAMPLE 1

A reactor A was continuously charged with 0.15 kmoles/hour of liquid chlorine and 0.185 kmoles/hour of liquid CH₃ Cl. This reactor was a perfectly stirred photochemical reactor operating under a pressure of 15 bar absolute and at 60° C. and cooled by water circulation; the reaction products had the following approximate composition (mixture of liquid+vapor):

    ______________________________________                                                   % by weight                                                          ______________________________________                                         HCl =       27.2                                                               CH.sub.3 Cl =                                                                              22.7                                                               Cl.sub.2    <0.5% of the mixture of the other                                              compounds                                                          CH.sub.2 Cl.sub.2 =                                                                        22.1                                                               CHCl.sub.3 =                                                                               22                                                                 CCl.sub.4 =6                                                                   ______________________________________                                    

A reactor B was continuously charged with 0.021 kmoles/hour of liquid chlorine and 0.043 kmoles/hour of liquid CHCl₃. This reactor was identical to the reactor A and operated under the same operating parameters. The reaction products therefrom had the following approximate composition (mixture of liquid+vapor):

    ______________________________________                                                     % by weight                                                        ______________________________________                                         HCl =         11.8                                                             Cl.sub.2      <1% of the mixture of HCL +                                                    CHCl.sub.3 + CCl.sub.4                                           CHCl.sub.3 =  38.5                                                             CCl.sub.4 =   49.7                                                             ______________________________________                                    

The two reaction mixtures were transferred into a distillation column operating under a pressure of 12 bar absolute, in which the HCl was separated from the other reaction products. A product having the following approximate composition was drawn off:

    ______________________________________                                                     % by weight                                                        ______________________________________                                         CH.sub.3 Cl = 22.2                                                             CH.sub.2 Cl.sub.2 =                                                                          21.7                                                             CHCl.sub.3 =  34.1                                                             CCl.sub.4 =22                                                                  ______________________________________                                    

This mixture thus obtained was then separated by successive distillation such that each of the chloromethanes was obtained, i.e., the equivalent of:

    ______________________________________                                                     kmoles/hour                                                        ______________________________________                                         CH.sub.3 Cl = 0.089                                                            CH.sub.2 Cl.sub.2 =                                                                          0.051                                                            CHCl.sub.3 =  0.058                                                            CCl.sub.4 =   0.029                                                            ______________________________________                                    

was thus obtained.

Thus, taking into account the recycling of 0.043 kmoles/hour of CHCl₃ into the reactor B, the production of higher chloromethanes was hence approximately:

    ______________________________________                                                     kmoles/hour                                                        ______________________________________                                         CH.sub.2 Cl.sub.2 =                                                                          0.051                                                            CHCl.sub.3 =  0.015                                                            CCl.sub.4 =   0.029                                                            ______________________________________                                    

which gave an amount, by weight of CH₂ Cl₂ and CHCl₃ in the CH₂ Cl₂ +CHCl₃ mixture, of, respectively, 70.3% and 29.7%.

EXAMPLE 2

The experiment of Example 1 was repeated, but an amount of CCl₄ corresponding to about 20% by weight of the chloroform was added to the chloroform charged to the reactor B. The only noticeable change in respect of the results obtained in Example 1 was the increase in unreacted chlorine in the reactor B.

EXAMPLE 3

Reactor A was operated under the same conditions as described in Example 1.

Reactor B was in this Example charged with 0.029 kmole/hour of liquid CH₂ Cl₂ and 0.05 kmole/hour of liquid chlorine. It operated under the same operating conditions as in Example 1. The reaction products had the following approximate composition (mixture of liquid+vapor):

    ______________________________________                                                        % by weight                                                     ______________________________________                                         HCl =            30                                                            Cl.sub.2         <95 of the total mixture                                      CH.sub.2 Cl.sub.2 =                                                                             2                                                             CHCl.sub.3 =     14                                                            CCl.sub.4 =      54                                                            ______________________________________                                    

After separation of the reaction products, the equivalent of:

    ______________________________________                                                     kmoles/hour                                                        ______________________________________                                         CH.sub.2 Cl.sub.2 =                                                                          0.053                                                            CHCl.sub.3 =  0.043                                                            CCl.sub.4 =   0.029                                                            ______________________________________                                    

was thus collected.

Taking into account the recycling of 0.029 kmole/hour of CH₂ Cl₂ to the reactor B, the production of higher chloromethanes was thus approximately:

    ______________________________________                                                     kmoles/hour                                                        ______________________________________                                         CH.sub.2 Cl.sub.2 =                                                                          0.024                                                            CHCl.sub.3 =  0.043                                                            CCl.sub.4 =   0.029                                                            ______________________________________                                    

which gave an amount, by weight of CH₂ Cl₂ and CHCl₃ in the CH₂ Cl₂ +CHCl₃ mixture, of, respectively, 27.8% and 72.2%.

While the invention has been described in terms of various preferred embodiments, the skilled artisan will appreciate that various modifications, substitutions, omissions, and changes may be made without departing from the spirit thereof. Accordingly, it is intended that the scope of the present invention be limited solely by the scope of the following claims. 

What is claimed is:
 1. A process for the simultaneous preparation of the higher chloromethanes CH₂ Cl₂, CHCl₃ and CCl₄, comprising chlorinating methyl chloride with chlorine in a first reaction zone A, chlorinating at least one of the higher chloromethanes CH₂ Cl₂ and CHCl₃ with chlorine in a parallel second reaction zone B, combining the reaction products from said first and said second reaction zones A and B, separating higher chloromethanes from said combined reaction products, and recycling at least one of said separated higher chloromethanes CH₂ Cl₂ and CHCl₃ as chlorination feed to said second reaction zone B, said chlorination feed being essentially free of methyl chloride.
 2. The process as defined by claim 1, said higher chloromethane feed in said second reaction zone B comprising a CCl₄ diluent.
 3. The process as defined by claim 1, the chlorination reactions in said first and second reaction zones A and B being carried out in liquid phase.
 4. The process as defined by claim 1, further comprising preliminarily gas scrubbing the higher chloromethane feed for said second reaction zone B.
 5. The process as defined by claim 1, further comprising scrubbing the reaction product issuing from said second reaction zone B with a higher chloromethane originating from said first reaction zone A, prior to combining said reaction products.
 6. The process as defined by claim 1, comprising separating said higher chloromethanes from said combined reaction products in a plurality of zones of separation.
 7. The process as defined by claim 6, said zones of separation comprising distillation zones.
 8. The process as defined by claim 1, further comprising recovering HCl and unreacted CH₃ Cl from said combined reaction products.
 9. The process as defined by claim 1, the temperatures in said first and second reaction zones A and B ranging from 50° to 120° C. and the pressures therein ranging from 10 to 50 bar.
 10. The process as defined by claim 9, wherein the ratio between the number of molecules of Cl₂ to the number of molecules of CH₃ Cl in said first reaction zone A ranges from 0.3:1 to 1.5:1.
 11. The process as defined by claim 10, said ratio ranging from 0.5:1 to 1:1.
 12. The process as defined by claim 1, further comprising recycling unreacted CH₃ Cl which comprises said combined reaction products, to said first reaction zone A. 